1. Field of the Invention
The present invention generally relates to power supplies and power factor correction power converters and, more particularly to reduction of common mode (CM) noise produced therein.
2. Description of the Prior Art
Most electronic devices currently in use operate on a more or less constant voltage direct current (DC) power rather than alternating current (AC) generally used for power distribution. This voltage is generally produced by a power supply or power converter included in such electronic devices which may include an AC to DC power supply such as a rectifier and filter and a further regulator stage to stabilize the output voltage at a desired value. It is also common, if not required, to provide an arrangement for correction of power factor and electromagnetic interference (EMI) filtering.
To improve efficiency of the voltage regulator generally included in electronic devices, a switching arrangement is often used to control the voltage developed at the output since an ideal switch does not consume power in the on or off state while significant power is dissipated by any regulating element or circuit which functions as a variable impedance in the current path since a voltage drop will always exist across it while significant current levels are provided through it. In some designs, such as alternators or for boosting the output voltage above the voltage supplied by a power source, switching is indispensable.
However, use of a switching arrangement is a source of common mode (CM) noise due to the unavoidable voltage variation dv/dt which results from the power switching function and the unavoidable parasitic capacitance between the power circuit and ground. CM noise is thus very difficult to control particularly due to the parasitic capacitance between respective portions of the power circuit and the case, the circuit board, other circuits and the like, which are subject to unpredictable variation from slight changes in geometry and proximity to various structures. Attempts to reduce CM noise have generally been attempted through strict symmetric design (which is not always compatible with requirements of the powered circuit and often requires additional components which may be unnecessary for any other reason) or through intensive filtering which also compromises efficiency and is often expensive and consumes significant space. Even so, reduction in CM noise has generally been limited to about 20 db which is insufficient to allow simplification or reduction in size and weight of EMI filtering arrangements.